gpio: Add gpio-charger to the documentation

The GPIO-based charger is another of the helpful devices built
on top of GPIO.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20200620201248.28843-1-linus.walleij@linaro.org